This invention relates in general to processes of producing magnesium metal. The two main methods for producing magnesium metal involve reduction of either the oxide or the chloride. The first of these, known as the Pidgeon process, requires a strong reducing agent (usually silicon or ferrosilicon) and high temperatures (well over 1000° C.). The second route, which until recently was the principal means of magnesium production, relies on the electrolysis of molten magnesium chloride (MgCl2), which forms molten magnesium metal at the cathode, and chlorine gas at the anode. The magnesium chloride required for the fused salt electrolysis can in turn be produced in several ways. The most direct method involves separation from seawater, or evaporation of natural brines, usually from salt lakes. Alternatively, magnesium chloride can be produced by treating magnesium carbonate (magnesite, MgCO3), magnesium oxide (magnesia, MgO), or a magnesium silicate (especially serpentine, Mg3Si2O5(OH)4), with aqueous hydrochloric acid, as in the Magnola process. A process developed by the Australian Magnesium Corporation involves removal of water from hydrated magnesium chloride (bischofite, MgCl2.6H2O) by azeotropic distillation with ethylene glycol, following which the magnesium chloride is precipitated from glycol solution as the hexammoniate by treatment with ammonia, and the ammoniate is decomposed to anhydrous magnesium chloride by heating. Yet another alternative process that avoids the need for the dehydration step is carbochlorination, whereby magnesium oxide is reacted with chlorine in the presence of carbon, producing magnesium chloride. A version of this reaction that involves magnesite in place of magnesia was the basis for the operation of a magnesium plant in Alberta, Canada. The direct electrolytic decomposition of MgO to Mg and O2, in a cell containing an oxide-ion conducting ceramic electrolyte, is also possible (the EIMEx process).
A common feature of current technologies for producing magnesium is the generation of carbon dioxide (CO2). Thus, production of magnesium chloride from magnesite by either calcination or acid treatment involves the evolution of one mole of CO2 per mole of magnesium chloride, and if dolomite is used there are two moles of CO2 produced; the same is also true for the carbochlorination of magnesite. Although the oxygen in the magnesia feedstock for the silicothermic reduction process is removed as silicon dioxide (SiO2), large quantities of CO2 are released in the production of the ferrosilicon reductant, and in the generation of the high temperatures that are required for the reaction to proceed.
Thus, it is desirable to develop an improved process of producing magnesium metal.